Re: [PATCH] KVM: SVM: Clear the CR4 register on reset

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 02, 2021, Babu Moger wrote:
> This problem was reported on a SVM guest while executing kexec.
> Kexec fails to load the new kernel when the PCID feature is enabled.
> 
> When kexec starts loading the new kernel, it starts the process by
> resetting the vCPU's and then bringing each vCPU online one by one.
> The vCPU reset is supposed to reset all the register states before the
> vCPUs are brought online. However, the CR4 register is not reset during
> this process. If this register is already setup during the last boot,
> all the flags can remain intact. The X86_CR4_PCIDE bit can only be
> enabled in long mode. So, it must be enabled much later in SMP
> initialization.  Having the X86_CR4_PCIDE bit set during SMP boot can
> cause a boot failures.
> 
> Fix the issue by resetting the CR4 register in init_vmcb().
> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>

Cc: stable@vger.kernel.org

The bug goes back too far to have a meaningful Fixes.

Reviewed-by: Sean Christopherson <seanjc@google.com>


On a related topic, I think we can clean up the RESET/INIT flows by hoisting the
common code into kvm_vcpu_reset().  That would also provide good motivation for
removing the init_vmcb() call in svm_create_vcpu(), which is fully redundant
with the call in svm_vcpu_reset().  I'll put that on the todo list.